Stephen G. Aanstoos, 20, a lifelong resident of the Washington area and a student at Purdue University, died at Fairfax Hospital Wednesday of crohn's disease, a disorder of the digestive tract.

Mr. Aanstoos was born in Arlington and with his family moved to Vienna. He was an honors graduate of James Madison High School in 1975. He was majoring in wildlife sciences at Purdue in Lafayette, Ind., in preparation for veterinary studies.

At Purdue he also worked at the Columbian Park Zoo in Lafayette. He exhibited animals and talked about them at schools and nursing homes there. He was engaged in a fund raising drive for the zoo when he became ill.

He was a member of the National Wildlife Federation and of the Friends of the National Zoo.

Survivors include his parents, Anthony Matthew and Frances H. Aanstoos, of the home in Vienna; five brothers, Christopher Michael, of Pittsburgh, Pa., Thomas Kevin, of Arlington, James Vincent, a student of Rice University in Houston, Tex., and Lawrence Matthew and Mark Anthony, both of the home, and two sisters, Antoinette Francine and Victoria Marie, both of the home.

The family suggest that expressions of sympathy be in the form of contributions in Mr. Aanstoos's memory to the Columbian Park Zoo in Lafayette.